Question: Evaluate $\dfrac13m-1-\dfrac12n$ when $m=21$ and $n=12$.
Answer: Let's substitute $ m= {21}$ and $ n={12}$ into the expression. $\phantom{=}\dfrac13 {m} -1 -\dfrac12{n}$ $=\dfrac13( {21}) -1 -\dfrac12({12})$ $=7-1-6$ $=0$